The invention relates to a device for the automatic actuation of a friction clutch in the transmission line of a motor vehicle between an internal combustion engine and a change-speed gear.
From U.S. Pat. No. 4,509,625 a device for the automatic actuation of a friction clutch in the transmission line of a motor vehicle between an internal combustion engine and a change-speed gear is known in which the clutch releaser of the friction clutch is positioned by an electric setting drive. The setting drive responds to an ideal-position signal of a control system which engages and disengages the clutch in dependence upon the working conditions of the motor vehicle. In starting driving in first gear or reverse gear the position of the clutch releaser is set in proportion to the rotation rate of the internal combustion engine. The higher the engine rotation rate is, the more the clutch is shifted, starting from a position of commencing torque transmission, into a position in which it is completely engaged. In this way the engagement of the clutch can be controlled by way of the position of the accelerator pedal. In the higher gears the clutch is engaged with predetermined setting speed of its clutch releaser. Since normally at higher rotation rates and at higher driving speed changing is effected into the higher gears, it is possible by predetermining a constant setting speed to obtain a sufficiently jerk-free clutch behavior in changing into a higher gear.
However problems can arise in changing down from a higher gear into the lower gear, since the friction clutch ordinarily has a progressive characteristic, that is there is no linear connection between the position of the clutch releaser and the torque transmitted by the clutch. In clutch engagement after changing down ordinarily the engine rotation rate is lower than the input rotation rate of the change-speed gear. If the clutch is engaged by means of the control system, then the engine will be accelerated by the gearing. According to the progressive characteristic of the clutch, the rotation rate increases initially only slightly, but finally very greatly. An undesired clutch engagement jerk occurs.
The present invention now shows a way in which such a clutch engagement jerk, occuring especially in changing down, can be avoided.